Abstracts

Rationale: Epilepsy is a disorder of brain microcircuits that arises from aberrant neuronal synchronization. Traditionally, this has been attributed to excessive excitation that is able to overcome inhibitory regulation by GABAergic interneurons within abnormal neuronal circuits. However, recent studies have raised the possibility that selective optogenetic activation of interneurons, can not only terminate, but also, under the right conditions, initiate, seizures in the mouse hippocampus. For example, optogenetic inhibition of PV interneurons can reduce the frequency of spontaneous epileptiform discharges in vitro. These findings inspired us to further investigate the role of parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP) interneurons during seizures.Methods: To facilitate temporally specific, on-demand seizure initiation, we developed a new in vivo optogenetic kindling model of ictogenesis by focally expressing ChR2 within excitatory neurons in the mouse neocortex and selectively activating them using a 473nm blue laser. We simultaneously carried out cell type–specific bulk calcium imaging in the contralateral cortex using Cre-dependent expression of the genetically encoded calcium indicator, GCaMP6f, within PV, SOM, and VIP interneurons, as well as excitatory neurons.Results: Using a novel stimulation pattern, we are able to reliably induce focal and generlized seizures, in both wild type and mutant mice. In these mice calcium imaging shows distinct, cell-type specific temporal patterns of activity relative to the onset and termination of electrographic seizure activity contralateral to the side of stimulation. In particular, changes in the activity of PV and SOM interneurons appear to be most highly correlated with the electrographic onset and termination of seizures. In comparison, increases in excitatory neuron activity are delayed with respect to the onset of the electrographic seizure, and changes in excitatory neuron activity are less tightly coupled to the termination of the electrographic seizure.Conclusions: In conclusion, we have developed an inducible in vivo optogenetic model for ictogenesis that makes it possible to study seizures that are triggered at very specific timepoints. Our findings suggest that GABAergic interneurons, specifically PV and SOM-expressing interneurons, may play important roles in both seizure initiation and termination. Surprisingly, increases in excitatory neuron activity do not appear to drive the spread of electrographic seizure activity. In the future, we hope to take advantage of our novel experimental setup to further elucidate the functional significance of various neuronal subtypes for seizure initiation, propagation and termination.